''Long ago, few faithful remained, while many gave themselves over to the black sorcery of Sauron's temple to Melkor at Armenelos. Thus deceitful Annatar spread a morbid obsession with death throughout Numenor, making the noblest among the races of Men resentful of Iluvatar's gift such that the more they clung to life, the more life slipped away from them. Eventually, through efforts ever more frantic, unnatural, and illicit, they brought down upon themselves a very world-shattering doom, as the tales speak of in the Akallabeth.''

I think Tolkien would have been fully aware of the meaning of the word he was using, so we have to assume he meant the Necromancer to have something to do with the dead. I always got this necromancy confused with Angmar and the Witch-King. I had these pictures in my mind of undead armies attacking the northern kingdoms.The Nazgul were certainly not living creatures. As Wraiths , one might regard them as dead, or perhaps more accurately undead creatures, ie creatures without life but animated by the dead spirits of the Men that they once were. Sauron was able to use the Nine Rings to bring them to this state and bind their dead (or undead) spirits to his will. Maybe it had something to do with a combination of the Nazgul's undead status and the Barrow-wights' connexion to that era. I bet he did practice a bit of Necromancy every now and then. The Nazgul have been mentioned, and remember that even the Witch King managed to recruit a bunch of Wights to infest the Barrow Downs. Exactly how is not made clear, elvish fea, victims of Morgul blade stabbings or old fashioned zombies, who knows?